RS-232 to RS-485 - sometimes you need the flexibility that a RS-485 gives you over the simple RS232!
|No handshaking is required to control the RS-485 driver. The handshake lines are required but only to provide power to the converter. The RS-485 driver is automatically enabled during each spacing state on the RS-232 side. During the marking or idle state, the RS-485 driver is disabled and the data lines are held in the marking state by pull-up and pull-down resistors.
The SCA-321 has an internal connection to prevent data transmitted from the RS-232 port from being echoed back to the RS-232 port. The SCA-321 is used as a two wire (half duplex) RS-485 converter with a third conductor to be used as signal ground. Using a common signal ground ensures that the common mode voltage between devices falls within the specifications of RS-485. This is especially important for networked devices with opto-isolated RS-485 interfaces.
Using the SCA-321 to communicate to most RS-485 devices does not usually require any termination. Termination is used to match impedance of a node to the impedance of the transmission line being used. When impedances are mismatched, the transmitted signal is not completely absorbed by the load and a portion is reflected back into the transmission line. This reflected signal can cause communication errors especially on long cable runs using a high data rate. If termination is required, the SCA-321 only supports AC coupled terminators. One terminator should be installed on each end of a straight cable run. A typical AC termination is a 120 Ohm resistor and a 0.1 uF capacitor in series between 485(+) and 485(-).